This invention relates generally to processes for improving the strength of laser-machined, articles formed of a silicon-based ceramic material and, more particularly, to such processes that utilize chemical etching.
Silicon-based ceramic materials such as silicon nitride (Si.sub.3 N.sub.4) are attractive materials suitable for structural use at temperatures exceeding 1000.degree. C., because of their high strength and hardness, high Young's modulus, low density, and high oxidation resistance. Ceramic materials such as silicon nitride also have a relatively low thermal expansion coefficient and high thermal conductivity, making them resistant to thermal shock. Silicon nitride can be prepared by hot pressing or hot isostatic pressing, and machining of some kind is frequently required to produce the desired final shape.
One promising technique for machining silicon-based articles utilizes a scanning beam from a CO.sub.2 laser. In the case of silicon nitride, the scanning beam heats the material to decompose it into gaseous nitrogen and liquid silicon, with most of the liquid silicon being ejected from the article's surface in the form of droplets. The beam is focused on the article's surface and moved at a generally constant speed, e.g., 200 to 250 centimeters per second, to produce a groove. Periodically moving the article and/or the beam transverse to the scan direction produces a number of overlapping, parallel grooves that together constitute the removal of a layer. The lens focusing the beam is periodically move downwardly so that a series of layers having controlled boundaries are removed, to produce the desired shape. One suitable apparatus for performing laser machining of this kind is described in copending and commonly-assigned U.S. patent application Ser. No. 268,770, filed Nov. 8, 1988, U.S. Pat. No. 4,914,270, and entitled "Method and Apparatus For Shaping Articles Using A Laser Beam." Laser machining of this kind is fast, e.g., one cubic centimeter removed in 200 seconds, and it produces smooth surfaces, e.g., three micron arithmetic average surface roughness, whereby it offers a promising alternative to more conventional diamond grinding for shaping articles formed of silicon nitride.
Although laser machining of silicon nitride articles has been shown to be effective and efficient, it has been found that the average strength of such laser-shaped articles can be 30 to 40 percent lower than that of correspondingly-shaped articles formed using diamond grinding. Although the scatter in strength values appears to be less for the laser-shaped articles than for the diamond-ground articles, the decrease in scatter appears to be insufficient to offset the decrease in allowable stress required by the loss in average strength.
It appears that the loss in strength results from the presence of small cracks in a thin silicon film remaining on the surface of the silicon-based ceramic substrate after laser machining. The film typically has an average thickness of about five microns. Internal stresses brought on by a thermal expansion mismatch between the silicon film and the underlying substrate are believed to cause the cracks to undergo a period of stable growth downwardly into the substrate when an external load is applied. Ultimately, when the external load exceeds a critical value, a failure will occur.
Prior techniques contemplated for removing or otherwise reducing the effects of such small cracks in the silicon film include annealing the articles (e.g., at 1100.degree. C.) in flowing wet oxygen to produce a silicon oxide film, and annealing the articles in dry nitrogen. Although both of these annealing techniques offer substantial improvement in strength, they are considered unduly time consuming and expensive. Another technique for reducing the undesired effect of small cracks in the silicon film is to diamond grind the affected surface to remove the silicon film and a portion of the underlying substrate. This procedure, likewise, is considered unduly expensive. Chemical etching of the laser-machined articles is not necessarily considered a viable alternative because of a concern that the etchant would attack the underlying substrate.
It should, therefore, be appreciated that there is a need for a more convenient and less expensive procedure for improving the strength of laser-machined articles formed of a silicon-based ceramic material such as silicon nitride. The present invention fulfills this need.